What Goes Under the Shingles? Underlayment Choices in a Grand Junction Roof Replacement

What actually goes down before the shingles go on? Most homeowners picture a roof as shingles nailed to wood, and that’s about it. But between the deck and the shingles sits a layer called underlayment, and in a roof replacement it does a quiet but important job: it’s the backup water barrier that keeps the deck dry when wind, hail, or age lets water sneak past the shingles. On the Western Slope, where a summer storm can drive rain sideways and hail can bruise shingles in minutes, that hidden layer earns its keep.

Let’s walk through a realistic example, then pull out the lessons so you know what to look for when you’re comparing quotes for a roof replacement.

A Redlands Homeowner Comparing Two Quotes

Say a homeowner out in the Redlands is getting bids after a hailstorm and lines up two estimates for the same 2,200-square-foot roof. The prices are close, within a few hundred dollars. But when they read the fine print, one quote lists “synthetic underlayment” and the other lists “15-pound felt.” They call and ask which one matters.

The honest answer: the underlayment line is one of the easiest places for two quotes to quietly differ, and it’s worth understanding before you sign. Felt, the old-school tar paper, has kept roofs dry for decades and still works. Synthetic underlayment is a woven plastic sheet that’s lighter, tougher, and less likely to tear or wrinkle. Neither is “wrong,” but they behave differently once they’re on your deck under the high-desert sun.

If you’re still deciding whether a full replacement is even the right call, our guide on when a Grand Junction roof is telling you it’s time is a good place to start before you get into material details.

Why the Difference Shows Up Fast Here

Here’s the part that trips people up. During a tear-off, the crew strips the roof down to the bare deck. There’s usually a window, sometimes a full day, where that deck is exposed or covered only by underlayment before shingles go on. In Grand Valley summers, that can mean baking sun one hour and a pop-up thunderstorm the next.

Felt handles this poorly. Left exposed, it can wrinkle, soak up moisture, and even tear if the wind picks up. Synthetic underlayment shrugs most of that off, which is why many roofers here prefer it, especially on jobs where weather is unpredictable. In our realistic Redlands example, the roofer who quoted synthetic explained that if an afternoon storm rolls in mid-job, the deck stays better protected.

That said, the shingles are still your main defense. Underlayment is the second line, not the first. So the goal isn’t to obsess over one line item; it’s to make sure your roof replacement includes a quality underlayment installed correctly, plus the right details in the spots that leak most.

The Spots Where Underlayment Really Earns Its Keep

The flat field of your roof, the big open slopes, is the easy part. The trouble spots are the edges and transitions:

• Eaves and valleys. These collect the most runoff. Many replacements add a self-adhering “ice and water” membrane here, a sticky, rubberized layer that seals around nails. Even in our dry climate, wind-driven rain and the occasional winter freeze make these zones worth extra protection.
• Around penetrations. Chimneys, vents, and skylights all interrupt the roof surface. Good underlayment work laps correctly around them so water is guided off, not under.
• Rakes and ridges. The edges where wind gets under shingles benefit from underlayment that’s fastened tight and won’t lift.

A realistic way to think about it: on a typical Grand Junction home, upgrading from basic felt to synthetic across the whole roof plus a proper membrane in the valleys and eaves might add a few hundred dollars to the job, not thousands. If you want to see how that fits the bigger picture, our Grand Junction roof replacement cost guide lays out where the dollars typically go.

What This Means When You’re Choosing

The lesson from that Redlands comparison is simple. Don’t just compare bottom-line prices; compare what’s actually being installed. Ask each roofer what underlayment they use, whether they add membrane at the eaves and valleys, and how they protect the deck if weather hits mid-job. A roofer who can answer those clearly is usually one who sweats the details that keep your roof dry for the long haul.

Underlayment pairs with your shingle choice, too. If you’re weighing options there, our breakdown of 3-tab versus architectural shingles covers the layer everyone sees, while this post covers the one they don’t. Both add up to how long the finished roof lasts, which ties directly into whether a new roof pays you back.

If you’d rather have someone look at your roof and explain your options in plain terms, you can get matched with a local Grand Junction roofer for a free inspection, or see the areas we cover across the Grand Valley.

Frequently Asked Questions

Do I really need to care about underlayment, or is that just upselling? It’s a real part of the roof, not a gimmick. You don’t need to memorize the specs, but knowing your replacement includes quality underlayment, and extra membrane at the leak-prone eaves and valleys, helps you compare quotes fairly instead of just chasing the lowest number.

Is synthetic underlayment worth the extra cost over felt? For most Grand Junction homes, the upgrade is modest, often a few hundred dollars, and synthetic holds up better to sun, wind, and a deck left exposed mid-job. Felt still works when installed and covered promptly. Ask your roofer which they’d recommend for your specific roof.

Can underlayment be replaced without redoing the whole roof? Not really. Underlayment sits beneath the shingles, so reaching it means removing them. That’s why it’s addressed during a full roof replacement rather than as a standalone repair. If your shingles are fine, the underlayment underneath usually is too.